The invention concerns an electric bus arrangement for DC-supply of power components, particularly for an inverter, having a first and a second plate, arranged in parallel with each other separated by an isolating layer, by which the first plate connects first connections of power components of a first group with first connections of power components of a second group, and the second plate connects second connections of the power components of the first group with second connections of the power components of the second group. Further, the invention concerns a method for minimising the inductance in an electric bus arrangement for DC-supply of switching power components, by which the current in a first plate of the bus arrangement flows in one direction and the current in a second plate of the bus arrangement, parallel to and arranged adjacent to the first plate, flows in the opposite direction.
In the following the invention is described on the basis of a frequency converter, in the following called inverter, even though it can also be used for other applications. Initially, such an inverter rectifies electrical voltage from the mains and provides it to an intermediary circuit as DC voltage. Normally, coil and capacitors are arranged in the intermediary circuit, which again is connected to a circuit arrangement, which produces a single- or polyphase AC-current through switching on and off switches. E.g. via the frequency the switching arrangement controls the speed of electric single- or polyphase motors. To limit the out-put losses of the inverter, a high switching frequency is required. A special case, concerns a simple inverter by which the DC-voltage comes from another source.
However, a high frequency switching means a heavy temporal current change, i.e. a high di/dt. Correspondingly, high voltage peaks are induced during the switching due to the inductivity of the bus arrangement. The induced voltage peaks result from the known relation v=L.multidot.di/dt. It is therefore very important to keep the inductivity L and thus the inductance of the bus arrangement as low as possible.
To keep the inductance low, the conductors of the bus arrangement should be as short, thin and wide as possible. When conductors with the same currents, however flowing in opposite directions, are arranged so that they lie very close to and overlap each other, the magnetic flux generated by the opposite currents can be almost eliminated. In total the magnetic flux around the conductors will be substantially zero. Thus current changes will only cause small flux changes, which drastically reduces the reactance or the inductance of the bus conductors.
It is therefore commonly known to laminate the bus arrangement, i.e. provide it with a positive conductor, an isolating layer and a negative conductor, which are arranged to overlap each other. These conductors or bus bars carry currents with the same amplitudes and opposite directions from and to the capacitor arrangement in order to eliminate the magnetic flux generated through the switching currents in the bus bars.
E.g. JP 62 040069 A describes a laminated bus bar arrangement with a fitted capacitor. The bus bar arrangement has legs or extensions connecting to the power components. However, these connection legs have different lengths, as the legs of the negative plate project by at least the thickness of the bus arrangement. Additionally, the capacitor is fitted on legs projecting from the bus arrangement, by which the capacitor is fitted on an edge of the bus arrangement, which requires more space.
A different bus arrangement is known from U.S. Pat. No. 5,132,896. Also here the bus bars are made as plates, i.e. a positive plate connecting power switch poles with capacitor poles, and a negative plate connecting the remaining power switch poles with the remaining capacitor poles. The negative and positive plates are separated by an isolating layer, and fitted on the power components by screws. A characteristic feature of this construction is that the bus bars are used for both current transfer and heat dissipation.
JP 04 133669 A shows a laminated bus bar arrangement with a positive and a negative plate, an isolating plate and an intermediary plate. This intermediary plate is used to connect two capacitors in series. For this purpose the intermediary plate is arranged in the same plane as the positive plate. The bus plates serve as connectors between the capacitor and a rectifier and as conductor between the capacitor and the switches, when the capacitors deliver their energy through the switches.
In prior art bus arrangements (in the following, "bus arrangement" must be understood as an arrangement of conductors), a high degree of flux minimising has already been obtained. However, large areas appear in the bus arrangements, in which the bus plates do not overlap each other and thus do not contribute to the flux reduction. These "blind" spots are found in the areas, in which the power components are connected with the bus plates. In the mentioned U.S. Pat. No. 5,132,896 the end of the negative plate is Z-shaped and is fitted on the first poles of the power components, while the end of the positive plate is also Z-shaped. The positive plate is fitted parallel and close to the negative plate. However, it projects over the negative plate to reach the second poles of the power components. As the positive plate is longer than the negative plate, an area appears, which can no longer be neglected, in which the compensation for the magnetic flux is missing, which occurs during the current transfer from the capacitor to the power components. Further, a magnetic flux is generated through an inductive current flowing transversely, viz. from one collector to another, which is not compensated. This missing compensation causes a limitation on the switching frequencies. As mentioned above, the parasitic reactances cause overvoltages exceeding the nominal data of the power components. This will lead to either a reduction of the life or even to a damaging of the power components.